Discontinuity of the Vlasov--Poisson Flow in $L_x^pL_v^\infty$

Abstract: We prove that solution maps of the Vlasov--Poisson equation are discontinuous at the zero initial datum in $X_p=L_x<sup>pL_v<sup>\infty$ for every $1\leq p&lt;\infty$, in $\mathbb{R}^{d} \times \mathbb{R}<sup>{d}$ with $d \le 3$, and for both attractive and repulsive interactions. For every $T&gt;0$, the trajectory of the solution in $L<sup>\infty([0,T];X_p)$ is discontinuous at zero, and for every sufficiently small fixed $t&gt;0$, the fixed-time map with values in $X_p$ is also discontinuous at zero. The counterexamples are smooth, nonnegative, bounded by one, and supported in a common compact subset of phase space. Their mass and initial $X_p$ norm tend to zero, whereas their solution norm at the observation time is at least one. The construction places narrow spatial packets on a lattice. Free transport allows a different velocity to select a packet at each spatial point, while a smallness estimate on the field ensures that the nonlinear characteristics are close to that of the free transport. For the same families, the phase-space $L<sup>q$ norms converge to zero uniformly in time for every $1\leq q&lt;\infty$.